Born in Portsmouth, Chamberlain joined the Southampton F.C. Academy at the age of seven and made his professional debut in 2010.

On 2 March 2010, he made his first team debut for Southampton, at 16 years 199 days, coming off from the bench in a 5–0 victory over Huddersfield Town and became the club's second youngest ever appearance maker behind Theo Walcott. Chamberlain later appeared as an 82nd minute substitute for Jason Puncheon on 8 May in the final game of the season against Southend United. At the beginning of the 2010–11 season, Chamberlain made his first competitive start on 10 August against Bournemouth in the first round of the League Cup. During the match he scored the second goal of Southampton's 2–0 victory, it was his first senior goal.

On 20 August, shortly after his 17th birthday, Chamberlain signed his first professional contract, which should last for three years. His first league start came in a 2–0 loss at home to Rochdale on 4 September. His first league goal came in a 2–1 victory over Oldham on 23 October 2010, which proved to be the match winner, and he scored two more goals and also got an assist in a 4–0 win over Dagenham & Redbridge on 2 November 2010, when he was voted Man of the Match in a league game for the first time.

He is the son of former Portsmouth and England International player Mark Chamberlain.His uncle, Neville Chamberlain, was also a professional footballer.

Arsenal manager Arsene Wenger will end the chase for the teenager rated the most talented in English football.Every major Premier League club, including Manchester City, Liverpool and Newcastle United, have been tracking Southampton boy wonder Alex Chamberlain.

But the 17-year-old midfielder visited Arsenal's training ground on Friday, met manager Wenger and will sign a £10million deal after talks with chief executive Ivan Gatzidis on Wednesday.Only a last-minute offer from City could disrupt the move but Chamberlain, the son of former England winger Mark Chamberlain, and his advisers will choose the club they believe will best help the teenager develop.

Arsenal's record in furthering the careers of England's Theo Walcott - also signed from Southampton as a teenager - and Welsh star Aaron Ramsey is understood to have convinced him.They have yet to finalise a fee with Southampton but Saints will not stand in Chamberlain's way.